Solid waste landfills contain enormous quantities of diverse waste materials. These materials decompose with time and produce a wide variety of decomposition products. It is common for solid waste landfills in this manner to generate large volumes of exhaust gas containing among other things, methane, carbon dioxide, hydrogen sulfide, and gases of many hydrocarbon and other organic compounds. These hydrocarbon and other organic compounds are well known air pollutants and are collectively referred to herein as volatile organic compounds or “VOC”.
Typical landfill exhaust gas contains high concentrations of methane and carbon dioxide, water vapor, and lesser concentrations of VOC's and other contaminants. Methane is commonly known as natural gas and is valuable commercial commodity as a combustible fuel for supplying energy and also as a raw material in many industrial significant processes. Thus it is very desirable from an economic viewpoint to capture the methane from landfill exhaust gas.
If landfill exhaust gas is not recovered, the methane escaping into ambient air presents a considerable source of air pollution. Accordingly, it is further desirable to prevent the methane from landfill exhaust gas for environmental protection purposes. Traditionally, landfill exhaust gas has been prevented from escaping to the environment by burning it in an open flame incinerator such as a flare stack. This process is inefficient. Consequently, a large fraction of the methane and other obnoxious contaminants in the exhaust gas survive to pollute the ambient air. Also, flare stack operation is a waste of the useful energy held by the methane in the exhaust gas.
Other conventional methods of recovering methane from landfill exhaust gas and other sources of crude natural gas have developed. These include gas separation processes in which the useful methane is separated from the other components of the source gas. Favored conventional gas separation processes typically utilize adsorption-regeneration technology in which the crude gas is passed through an adsorbent material that rejects selected components of the crude and rejects others. For example, pressure swing adsorption (“PSA”) or Thermal Swing Adsorption (“TSA”) technologies involve selectively adsorbing contaminants of crude gas onto adsorbent particles and allowing the so-called sweetened gas to pass through the PSA/TSA units.
Unfortunately, the adsorbent particles ultimately become saturated with the contaminants and lose ability to adsorb beyond a maximum amount. Before more contaminants can be removed from the crude, the adsorbent particles must be regenerated. This normally involves exposing the saturated particles to high temperatures, and fluids that have low concentrations of the contaminants to promote desorption of the contaminants from the particles. For example, TSA requires a supply of high pressure steam and PSA requires a supply of clean, usually low pressure gas. Additionally, adsorption-regeneration technology normally also requires support facilities for removal of water vapor, and pre-conditioning the crude gas, e.g., by compressing it to high pressure. Thus it is very costly in financial and energy consumption aspects to operate conventional adsorption-regeneration technologies for recovering useful methane from crude natural gas and landfill exhaust gas.